1. Field of the Invention
The present invention relates to a printing apparatus and a printing method capable of correcting a transport amount of a print medium when an image is printed on the print medium with transporting the print medium.
2. Description of the Related Art
In recent years, printing apparatus such as printers are finding a growing range of applications in printing picture images. Particularly, ink jet printers have achieved a significant progress in reducing the size of ink droplets ejected from a print head and thus are able to print images equal to or higher in image quality than silver salt pictures. In such ink jet printers, an image is formed on a sheet (print medium) by transporting the sheet along a transport path passing through a print position that faces the print head and ejecting ink onto the sheet being transported from the print head kept out of contact with the sheet.
To print an image up to a rear end area of the sheet with respect to the transport direction, another transport means (downstream transport means) needs to be provided on a downstream side of the sheet transport direction in addition to a separate transport means (upstream transport means) installed on an upstream side of the sheet transport direction. The upstream and downstream transport means may, for example, be a construction including rollers that hold the sheet in between as they rotate. If the transport means are installed on the upstream and downstream sides of the transport direction, the sheet is first transported only by the upstream transport means. Then, after the front end of the sheet reaches the transport position of the downstream transport means, it is transported by both of the upstream and downstream transport means. After the rear end of the sheet gets out of the transport position of the upstream transport means, it is transported only by the downstream transport means.
In the construction in which the sheet is transported in the sheet transport direction by the upstream and downstream transport means, the print position of an image on the rear end area of the sheet may deviate, degrading an image quality. That is, as the sheet gets out of the upstream transport means, the transport amount of the sheet may greatly vary from a predetermined value, as by backlash of gears in a drive system of the rollers included in the upstream transport means, shifting the print position of the image and degrading the image quality. When the sheet is being transported only by the downstream transport means after the rear end of the sheet has come out of the upstream transport means, a sheet transport accuracy becomes lower than when the sheet is transported by both of the upstream and downstream transport means, making the image print position more likely to deviate, giving rise to a problem of image degradation.
Japanese Patent Laid-Open No. 2002-254736 proposes a method for preventing image degradations in a rear end area of the sheet caused by such a deteriorated transport accuracy in a serial scan type ink jet printing apparatus. This method involves reducing the transport amount before the rear end of the sheet comes out of the upstream transport means situated on the upstream side of the transport direction and, when the rear end of the sheet leaves the upstream transport means and the transport amount of the sheet changes, changing a amount the downstream transport means transports the sheet. Further, a range of nozzles in the print head that are allowed to be used is limited and the positions of the nozzles in operation are shifted at the same time that the transport amount of the sheet is changed.
Further, the transport amount when the sheet is transported only by the downstream transport means is set equal to the transport amount when the rear end of the sheet leaves the upstream transport means so that it is smaller than when the sheet is transported by both the upstream and downstream transport means.
With the construction described in Japanese Patent Laid-Open No. 2002-254736, the image degradations in the rear end area of the sheet can therefore be minimized by reducing the sheet transport errors. That is, when the rear end of the sheet leaves the upstream transport means or during the transporting of the sheet after it has left the upstream transport means, possible reductions in the transport accuracy can be minimized.